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I ran diskutil list to get a list of drives on my system, taking note of the drive number of the drive named EFI and the other one named BOOTCAMP, which in my case, the EFI drive was 1 and the BOOTCAMP drive was 3

diskutil unmount /Volumes/BOOTCAMP
sudo chmod 777 /dev/disk0s1 <--- This is the drive no. of the EFI disk
sudo chmod 777 /dev/disk0s3 <--- This is the drive no. of the BOOTCAMP disk
sudo chmod 777 /dev/disk0

sudo VBoxManage internalcommands createrawvmdk -rawdisk /dev/disk0 -filename win10raw.vmdk -partitions 1,3
sudo chmod 777 win10*

ls -l
-rwxrwxrwx 1 root staff 37376 Sep 16 18:22 win10raw-pt.vmdk
-rwxrwxrwx 1 root staff 754 Sep 16 18:28 win10raw.vmdk

http://web.archive.org/web/20181103074214/http://danielphil.github.io/windows/virtualbox/osx/2015/08/25/virtualbox-boot-camp.html